German Patent Publication DE 100 05 491 A1 describes a device for measuring the electrical conductivity of liquids using an inductive method in which a closed conduit made of non-conductive material is provided which can be traversed by fluid flow. This conduit is in the shape of an “8”, and the loop is routed twice through ring cores. The conduit is traversed by a liquid to be measured, and an AC voltage is applied to a first coil, thereby inducing a voltage in a second coil, the induced voltage being usable to determine the conductivity of the fluid. Such devices are usually manufactured using chip-cutting methods, which involves a high level of manufacturing complexity and, in addition, results in high flow losses, which have to be compensated by a higher pumping capacity.
Sensor devices in which immersion sensors are provided which are integrated into an existing piping system can also be used to measure the conductivity of liquids. However, such sensor devices measure not only changes in conductivity in the fluid, but rather in the entire circulation system. When used in dishwashers, such sensor devices produce different measurement values due to the fact that dishwashers are loaded with conductive utensils made of metal and with non-conductive items.